To the Editor.—The recent article, "Zinc Lowers High-Density Lipoprotein [HDL]-Cholesterol Levels," by Hooper et al (1980;244:1960) supports the hypothesis of Klevay1 that relative or absolute copper deficiency may contribute to elevated serum cholesterol level and to the occurrence of atherosclerotic cardiovascular disease in humans. Zinc is the principle nutrient that causes a relative deficiency of copper. Pharmacologic intakes of zinc have been shown to induce copper deficiency in humans (1978;240:2166). The mechanism appears to be a suppression of intestinal absorption of copper (1978;240:2188) and, perhaps, an increased biliary excretion of copper. Copper deficiency in rats causes an increase in plasma low-density lipoprotein-cholesterol and a relative decrease in plasma HDL-cholesterol.2 Similar abnormalities in serum cholesterol level were observed in one human subject who was experimentally deprived of copper for 16 weeks by feeding a mixed diet containing 0.8 mg of copper daily, a level that did not induce